An electron, in addition to being characterized by its energy and momentum, is characterized by a quantity known as its spin. Quantum mechanical principles limit the possible values of the spin of an electron to plus(+) or minus(-) h/(4pi), where h=Planck's constant. The plus or minus designations depend on whether the electron is oriented parallel (or "up") or antiparallel ("down") to a pre-selected arbitrary direction. A beam of electrons necessarily contains a large number of electrons where each individual electron contributes a spin, either up or down. If a greater population of one electron spin type exists in the beam, then the beam is spin polarized. The spin polarization of the beam is quantifiable by the following equation: EQU P=(N.sub.up -N.sub.dn)/(N.sub.up +N.sub.dn),
where N.sub.up electrons have their spins in the up direction and N.sub.dn spins are pointing down. P represents the net orientation of the beam or ensemble of electrons. P values may range from +1 (all spins up), to 0 (an equal number of spins up and down), to -1 (all spins down).
As noted, the preselected "up/down" polarization axis is arbitrary as to where it is oriented relative to the coordinate system used for measurement. It is preferred, for purposes of interpretation, to employ a cartesian coordinate scheme where the determination of the cartesian components of the polarization of the electron beam, P.sub.x, P.sub.y, and P.sub.z, is equivalent to the determination of the magnitude and direction of the polarization vector. A beam with vector polarization is further characterized as transversely polarized when the polarization is perpendicular to its velocity and longitudinally polarized, when the vector polarization is parallel to its velocity direction.
Known sources for producing beams of polarized electrons include photoemission from GaAs, described in U.S. Pat. No. 3,968,376; and secondary emission from a ferromagnet, as discussed by Unguris et al in The Physical Review Letters 47, 72(1982). Because an electron has a spin of h/(4pi) and this spin has an associated magnetic moment of one Bohr magneton, a measurement of the electron spin orientation yields information about, for example, the origin of the electron or its interactions. An example of the uses of electron spin polarization for analytical applications includes the scattering of very high energy electrons (10.sup.10 eV) from other elementary particles to test the violation of the physical law of parity conservation. Another use involves measurement of the interaction characteristics of an electron with an isolated atom, including measurement of the polarization of the electron before and after scattering. Such measurement demonstrates the extent of electron substitution within the atom and/or the degree of electron deflection from the atomic charge cloud. Additionally, spin polarization measurements may be used for probing solid materials in the case of ferromagnetic materials due to the fact that electrons emitted from a ferromagnetic material either by photoemission, field emission, or secondary emission can be used to characterize the magnetic nature of the material.
Information storage mechanisms now exist in which magnetic domains in ferromagnetic materials are oriented parallel or antiparallel relative to particular reference coordinates. The development of high density, magnetic storage mechanisms is contingent upon the microscopic investigation of magnetic domains which is enhanced by the use of electron polarization detection. Information so stored can be "read" by measuring the spin polarization of electrons emitted therefrom. Where a finely focused electron beam, e.g. a scanning electron microscope, generates electron emission from the magnetic domain of the information storage device, the spin polarization of the emitted electrons can be measured. Hence, it is possible to read information stored in much greater density, than, for example, can be used with optical microscopy.
The traditional means of measuring electron spin polarization is known as Mott scattering, after Sir Neville Mott. In 1932, Mott predicted theoretically that the scattering of an electron from an atom would depend upon the direction of the incident electron's spin if (1) the electron was scattered through an angle greater than 90 degrees as measured with respect to the incident beam direction, (0 degrees=forward scattering, 180 degrees=backscattering along the beam); (2) the nucleus from which the electron scattered had a high atomic number Z; and (3) the speed of the electrons approached the speed of light, that is, electrons with energies greater than 50,000 electron volts. Under these conditions, there is an interaction between the spin of the incident electron and the angular momentum associated with the trajectory of the electron scattering from the nucleus. If the incident electron beam has a spin polarization normal to the scattering plane, there is an observable difference in scattering probability for scattering at the same angle to the right or the left.
From this theory, the Mott polarization detector emerged. The Mott detector measures the backscattered intensity for equal angles to the right and left of the beam's incident direction to determine the degree of polarization normal to the scattering plane. The normal component to the scattering plane is defined as EQU P.sub.n =A/S, with A=(I.sub.L -I.sub.R)/(I.sub.L +I.sub.R),
where A is the scattering asymmetry; I.sub.R and I.sub.L are respectively, the scattered electron intensities to the right side of the beam and the left, and S is the analyzing power which depends on the scattering target and the particular scattering geometry. Accordingly, the Mott detector employing a thin gold foil target in the thickness range of 300-1500 Angstroms, is calibrated to determine the effective analyzing power, S, by using theoretical calculations of the spin asymmetry in scattering 100,000 to 120,000 electron volt electrons.
To avoid the multiple scattering or absorption of elecltrons within the foil, since such effects are not accounted for in the theoretical model used to calibrate the detector, the Mott detector employs very thin gold foils for the target. The calibration of the detector involves a series of measurements of polarized electron beam scattering for well-defined geometries as a function of the target foil thickness.
The Mott device suffers reduced efficiency as a result of many of the incident electrons passing through the scattering target, i.e. the target is not opaque to the electron beam. At 100,000 electron volts, the target is not opaque to the high energy electrons where the mean free path of the electrons is sufficiently long so that many pass through the thin gold foil. As a result of using thin foils to minimize scattering typically only about 0.01% of the electrons entering the device are back scattered into the detectors actually used in the measurement. In contrast, low energy electrons (with energies less than a few thousand electron volts) are much more strongly interacting and are stopped by a gold foil of 300 Angstroms. The target opacity depends on both the electron energy and the target density.
There are a number of additional disadvantages of the Mott spin analyzer for measurement of electron spin polarization: (1) the intensity asymmetry occurs over a range of scattering angles where the scattering cross section is quite small, resulting in small signals to be detected. (2) The apparatus is very bulky owing to the high voltage isolation and safety region required for 100,000 volt operation, and (3) The analyzer and detectors are not readily movable to allow measurement of polarization of electrons being emitted by different sources or a source in different directions.
The next advances in this art evolved during the 1960's when it was discovered electron scattering at low energies (&lt;1000 electron volts) from a mercury atomic beam was possible. At such energies, the scattering is from the entire atom (nucleus plus electrons) rather than from the nucleus alone. Although avoiding the apparatus problems associated with the high voltage Mott Detector, the mercury atomic beam provides a target of significantly lesser density than that provided by the Mott gold foil target. Consequently, the scattering efficiency is very small. Furthermore, these devices generate a small angular range over which significant scattering asymmetry occurs and a minimal scattered intensity. An additional physical consideration, somewhat disadvantageous, is based on the necessity to employ large vacuum pumps to maintain sufficiently low vacuum in the presence of a mercury atom beam. Moreover, depending on the application, the mercury beam is often corrosive. Also, like the Mott Detector, the mercury atomic beam suffers detection inefficiencies caused by the target not being opaque to the electron beam. These inefficiencies are caused, in the case of the mercury atomic beam, by low target density.
A more recent development in spin polarization analysis was introduced by Kirschner and described in U.S. Pat. No. 4,153,844. The Kirschner detector is distinguishable from the above-described devices as it employs an opaque monocrystal scattering target for determining electron spin dependent interactions. The spin dependent interaction is the spin orbit interaction in the scattering of the incident electrons from the monocrystal. The monocrystal presents an orderly arrangement of atoms in crystal planes which, upon electron beam impingement, causes the electrons to be diffracted backward into well defined beams. These beams of scattered electrons are symmetrically spaced relative to the electron beam incident on the monocrystal surface. Significant multiple scattering occurs which cannot be reliably calculated thereby necessitating experimental calibration.
While the Kirschner device is compact and allows operation at low voltages, it has several disadvantages:
(1) The actual intensity of any two symmetrically diffracted beams which would be used for a polarization measurement is small, typically 0.1% of the incident beam.
(2) The angular spread of the incident electron beam at the monocrystal target is required to be less than two degrees, which in turn limits the variety of electron beams which can be analyzed. Being dependent on diffraction from the crystal planes, the device severely constrains the relative alignment of the incident electron beam, the planes of the atoms in the monocrystal, and the position of the detectors for the backscattered electrons. Where impingement of electrons is not perpendicular to the monocrystal, those electrons are diffracted in different directions, and consequently, are no longer symmetrically displaced about the incident beam. Thus, the Kirschner detector provides a minimal angular range for significant scattering asymmetry suitable for spin polarization detection.
(3) The angle of diffraction also changes with incident electron energy and at an incorrect energy, the diffracted beams may be shifted off the detector. In order to employ the monocrystal as a detector, the spread of energies of the electrons in the incident beam, is necessarily less than 5 electron volts. Furthermore, the detector's analyzing power decreases rapidly when the beam energy changes from the optimum energy and may even change sign leading to inaccurate or erroneous results.
(4) Due to very sensitive, surface diffraction, a Kirschner monocrystal surface must be cleaned to be free from any contaminants and maintained in ultrahigh vacuum. Such cleaning a surface involves heating to high temperatures in ultrahigh vacuum (&lt;10.sup.-9 Torr), treating with specific gases at specific temperatures, or bombarding with rare gas ions followed by an annealing process. For example, in Kirschner, a tungsten monocrystal is cleaned by heating to 1800.degree. K. in 10.sup.-6 Torr of oxygen for a period of order 10 minutes every few days. Between these treatments the crystal surface must be cleaned periodically (every 15-30 minutes) by heating rapidly to 2500.degree. K. in vacuum.
The analyzing capability of the above-described detectors may be arithmetically quantified in a figure of merit, FM=S.sub.2 I/I.sub.o, when the statistics of the measurement are the limiting factor. Thus, it is desirable to maximize S, the analyzing power, as well as to maximize the fraction representing the intensity of the scattered electrons I measured in the collector divided by incident electron beam intensity I.sub.o. With reference to the figure of merit, the angular and energy spread are not included in the conventional calculation, FM=S.sup.2 I/I.sub.o, because only the I.sub.o reaching the target is applicable. In general applications with the Kirschner device, it is necessary for the electron optics to reject much of the electron beam before it reaches the target surface so as to satisfy the severe constraints on spread in angle and energy. Consequently, the I.sub.o at the target is much smaller and the measurement signal is severely decreased. Even though a figure of merit of 10.sup.-4 can be achieved for the Kirschner monocrystal analyzer, under optimal conditions, the monocrystal device is efficient only for well collimated beams with narrow energy spread. Moreover, the monocrystal device requires the elimination of any inelastically scattered electrons, (usually those which have lost over 10 electron volts of energy in the scattering process) which results in a loss in useful scattering intensity.
In view of the foregoing, it is evident that there has long been a need for a simple, efficient and compact spin polarization detector.